Abstract

Archean subaqueous basalt flows in Rouyn–Noranda area consist of several structural divisions in vertical sequence. The complete, ideal sequence of structural divisions within a typical flow is, from base to top: massive lava, pillow lava, isolated- and broken-pillow breccia (pillow breccia), and stratified hyalotuff. Any variant of this standard sequence, formed by omission of one or several of the structural divisions, has been observed. Most commonly encountered are the following four sequences: (1) massive lava – pillow breccia, associated in the field with the sequence pillow lava – ± pillow breccia, and (2) massive lava – pillow breccia – hyalotuff, associated with the sequence pillow lava – pillow breccia – hyalotuff.Lateral transitions from massive lava into pillow lava have been mapped and suggest that pillow lava is distal relative to massive lava. Large flow lobes or tubes (megapillows) are present in the transition zone. Mapping of the relations of megapillows and normal-size pillows suggests that pillows form by a budding and branching process, rather like present-day pillows observed to form during the flow of lava into the sea. Relations between massive lava, pillow lava, and pillow breccia suggest that massive lava formed by the surging advance of large volumes of hot lava of low viscosity; pillows formed at the distal flow front due to a decrease of temperature and an increase of viscosity, resulting in a decrease of the flow velocity; pillow breccias apparently formed at the waning stage of the eruption, when the supply of fresh lava was reduced. Hyalotuff apparently formed by lava-fountaining at the fissure, in shallow water.

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